Technical Field
The present invention relates to pseudo-random bit sequence (PRBS) generators, particularly for generating multi-level test signal for input/output (I/O) circuits.
Description of Related Art
Multi-level signaling is commonly known as multiple pulse amplitude modulation (PAM), or multi-PAM, and has been implemented with radio or other long-distance wireless signaling systems. The use of multi-level signaling for communication between devices in a closed system is relatively new, as in the past the short distance transmission of binary signals could be accelerated easily by increasing the transmission frequency. As the raise of the transmission frequency comes to its limits, multi-level signaling is an option. Using multi-level signals instead of binary signals increases the data rate of a data transmission system independent of the transmission frequency, so that, with applying multi-level signaling, the signal frequency of the system does not need to be increased.
Hence, digital I/Os which allow multi-level signaling have gained more and more importance in the last couple of years. As any other system components, parts of the system operating with multi-level signaling need to be tested, and this testing is often based on pseudo-random test signals. The testing of multi-level I/Os of devices, however, is difficult, since conventional testing equipment is typically merely providing binary test signals.
Testing equipment comprises pseudo-random binary signal (PRBS) generators which are commonly based on linear feed-back shift registers. Such PRBS generators are designed for NRZ (non-return to zero) or 2-PAM pattern and cannot provide multi-level signaling due to the concept of generation.
Recently, multi-level signaling has been generally used for inter-device communication, particularly in digital-to-analog (DAC) transmitters (Tx) and analog-to-digital (ADC) receivers (Rx), Tomlinson-Harashima precoders and the like. To provide a sufficient and appropriate testing quality/coverage of such systems, a multi-level PRBS pattern needs to provide equally distributed symbols even for a relatively short pattern length and to provide patterns which are compatible with NRZ PRBS checkers available in commonly used measurement equipment. NRZ PRBS checkers are usually configured to detect a binary level and to provide no clock boundary crossing between a PRBS generator and an I/O because of simulation time issues and re-timing of clock boundaries.
A general approach to apply a straightforward multi-level PRBS pattern, by assigning subsets of bits to a multi-level symbol, results in a pattern which is not compatible to NRZ-based PRBS checkers in commonly used measurement equipment. Since a NRZ (non-return to zero)-coded pattern generally assigns a specific state of the transmission path to every transferred symbol, the states of the signal can be distinguished by standard comparators of conventional PRBS checkers.
Furthermore, for short test pattern lengths, the symbol generated by assigning the subsets of bits to a multi-level symbol are no longer uniformly distributed. Uniform distribution is one of the requirements for proper testing of I/Os by means of a conventional PRBS checker.
Furthermore, in feedback loop timing limited systems, the pattern generator needs to be operated at a much faster clock rate and hence clock boundary crossing and random-logic-macro (RLM) timing become an issue.
From document U.S. Pat. No. 7,162,672 B2, it is known that error detection mechanisms for devices that have multilevel signal interfaces test multilevel signals of an interface with a binary test apparatus. The error detection mechanisms include converting between multilevel signals of the interface and binary signals of the test apparatus.
In document US 2014/0003805 A1, a method for generating multilevel pseudo random symbol sequences for testing a transmission system is disclosed. The method employs n-th bit tapping mechanisms, where said method is comprised of generating pseudo random symbol sequences employing a linear feedback shift register; checking if said symbol sequences satisfy a pre-defined condition; and tapping bits to said symbol sequences as per said condition for producing a multilevel pseudo random symbol sequence to be fed to said transmission system.
Furthermore, in US 2006/0242483 A1, error detection mechanisms for signal interfaces, including built-in self-test (BIST) mechanisms for testing multilevel signal interfaces, are disclosed. BIST mechanisms may include test signal generators and mechanisms for determining whether the test signals generated are accurately transmitted and received by the interface. The BIST mechanisms may check a single input/output interface, a group of interfaces or may operate with a master device that tests a plurality of interfaces by sending test signals for storage by and retrieval from one or more slave memory devices.
Mahendra Sakare et al., “A 4×20 GB/S 29-1 PRBS GENERATOR FOR TESTING A HIGH-SPEED DAC IN 90NM CMOS TECHNOLOGY”, Progress in VLSI Design and Test Lecture Notes in Computer Science Volume 7373, 2012, pp. 252-257, discloses a pseudo-random binary sequence (PRBS) generator to test a 4-bit digital to analog converter (DAC). The architecture results in generation of four de-correlated sequences from a single 9-bit LFSR (linear feedback shift register) with minimal circuitry. The PRBS and DAC can together be used for generating pseudo-random multi-level test symbol sequences for high-speed communication
In another document, Karout, J., “CMA MISCONVERGENCE IN COHERENT OPTICAL COMMUNICATION FOR SIGNALS GENERATED FROM A SINGLE PRBS”, Dept. of Signals & Syst., Commun. Syst. Group, Sweden, Wireless and Optical Communications Conference (WOCC), 2011, 20th Annual, 15-16. April 2011, generating multilevel signals using a single pseudo-random binary sequence is disclosed.
From document Jikyung Jeong, “A CMOS 3.2 GB/S 4-PAM SERIAL LINK TRANSCEIVER”, Dept of Electron. Eng., Sogang Univ., Seoul, South Korea, SoC Design Conference (ISOCC), 2009, International, a multilevel pulse amplitude modulation (PAM) transceiver is known. By using 4-PAM signaling, symbol rate is effectively decreased compared to the binary signaling.
It is an object of the present invention to provide a multi-level PRBS generator for generating a multilevel test signal.